Ventilation of H1N1 Severe Respiratory Disease with Airway

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Ventilation of H1N1 Severe Respiratory Disease
withAirway Pressure Release Ventilation(APRV)
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Problem with Conventional Ventilation Strategies

• Most time spent at baseline level (set PEEP)
• This level may not be sufficient to recruit lung
  units
• During tidal ventilation lung units are recruited
• MAP a factor critical for good oxygenation
  remains low

Vol
Conventional ventilation spends most time here
Pres
Insp
Exp
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Airway Pressure Release Ventilation Strategy

• Fundamental concept of APRV
• maintain optimal V/Q by optimising MAP
• Ventilates from point much higher on PV curve
• Maximizes the recruitable surface of the lung

Vol
APRV spends most time here
Conventional ventilation spends most time here
Pres
Insp
Exp
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Airway Pressure Release Ventilation Strategy
APRV takes advantage of the collateral channels
of ventilation that are barely used at the FRC
level in normal, healthy lungs
In diseased states they become important
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What is APRV?
? Best described as continuous positive airway
pressure (CPAP) with regular, brief, intermittent
releases in airway pressure.
Technically, APRV is a time-triggered,
pressure-limited, time-cycled mode of mechanical
ventilation.

• The CPAP level drives oxygenation,
• The timed releases aid in CO2 clearance.

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When is APRV the mode of choice?
It is best suited as the primary mode of choice
for patients with acute lung injury
? Airway pressure release ventilation was
designed to oxygenate and augment ventilation for
patients with ALI or low-compliance lung disease
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How does APRV work ?

• APRV commences at an elevated baseline pressure
  (similar to a plateau pressure) and follows with
  a deflation to accomplish tidal ventilation
• Spontaneous breathing may occur at either the
  plateau pressure or deflation pressure levels.

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How does APRV work ?

• Airway pressure release ventilation begins on
  the pressure-volume curve between the lower and
  upper inflection points
• And
• uses a release, not an increase, of pressure from
  its baseline.

Therefore, oxygenation and ventilation occur
predominantly within the upper and lower
inflection points
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Rational of APRV

• Sustained plateau pressure
• promotes alveolar recruitment while being
  maintained at an acceptable level.
• The number of respiratory cycles is minimized
• prevents both the repetitive opening of alveoli
  and alveolar stretch, that may result in lung
  injury.
• APRV can unload inspiratory muscles
• decrease the work of breathing associated with
  chronic obstructive pulmonary disease.

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Airway pressure release ventilation recruits lung
units by optimizing end-inspiratory lung
volume
( Ideally, the end-inspiratory pressure, which
equates to P High or plateau pressure, should be
kept beneath 35 cm of water pressure. )
This protective lung strategy has several
positive effects

• The preset pressure limit prevents or limits,
  over-distension of alveoli and high-volume lung
  injury.
• APRV affects tidal ventilation by decreasing
  rather than increasing airway pressure.
• ( Decreasing lung volume for ventilation further
  limits air space over-distension and the
  potential for high-volume lung injury. )
• Maintaining airway pressure
• optimizes recruitment
• prevents or limits low-volume lung injury by
  avoiding the repetitious opening of alveoli.

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Settings in APRV
Only FOUR essential parameters to set
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PHigh

• Convert the plateau pressure of the conventional
  mode to PHigh
• Maximize V/Q relationship and recruitment thru
  MAP
• Aim for expired minute ventilation of 2 to 3
  L/minute ( gt than on CV )
• Lower PHigh slowly if pathological signs of
  elevated MAP are noted
• Increased release volume
• Decreased SpO2
• Increased heart rate

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THigh

• THigh is set for maintaining CPAP at a minimum
  of 4.0 seconds.
• Important for maximising recuitment
• The goal is to create a nearly continuous airway
  pressure level, which serves to recruit collapsed
  alveoli and maintain recruitment, thus optimizing
  oxygenation and compliance.
• Determines the set ( mandatory ) frequency
• frequency should always be less than 12
• THigh of less than 4.0 seconds begins to impact
  mean Paw negatively.
• As a patients lung mechanics improve, THigh is
  progressively lengthened to 12-15 seconds,
  usually in 0.5 to 2.0 second increments.
• Patient can breathe spontaneously during APRV

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PLow

• PLow is set at 0 cm of water pressure
• The PLow of zero is selected because minimal
  resistance to exhalation is the goal.
• Higher pressures may impede expiratory gas flow
  during passive lung recoil.
• The valid concern of collapsing alveoli with a
  PLow of zero is negated with the use of a short
  T Low (0.50.8 seconds) to maintain end
  expiratory lung flow and volume.

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TLow

• TLow is set between 0.5 and 0.8 seconds for
  release of pressure
• Time should be set by observing the release flow
  pattern
• Release should end when flow reaches bet. 50 25
  of peak expiratory flow

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TLow

• TLow depends on expiratory time constants (T),
  which are a product of the compliance of the
  respiratory system and the resistance of the
  airways
• Low-compliance states, such as ARDS, will have
  lower (or shorter) expiratory time constants and
  therefore a lower (or shorter) T Low.
• High resistance diseases, such as asthma, have
  longer time constants and require longer release
  times

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TLow

• Optimal release time ( TLow ) allows for
  adequate ventilation while minimizing lung volume
  loss.
• Essentially, release time ( TLow ) should impede
  complete exhalation in the slower compartments of
  the lung (i.e., areas of high compliance or high
  resistance to exhalation) and generate regional
  intrinsic PEEP.

Theoretically, this will enhance alveolar
recruitment
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TLow

• An excessively long TLow encourages
• alveolar derecruitment,
• atelectasis,
• airway closure during the release phase.
• An insufficient TLow potentially may result in
• inadequate exhalation, leading to dead space
  ventilation,
• hypercapnia,
• hemodynamic compromise

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Airway Pressure Release Ventilation
Time Triggered Time-cycled Ventilation
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Airway Pressure Release Ventilation
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Releases
1 2 3
4 5 6
7 8
-20
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APRV (Airway Pressure Release Ventilation)
Spontaneous breaths
CPAP Released
CPAP Restored
CPAP Level
Airway Pressure
CPAP Level 1
CPAP Level 2
Time
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Weaning in APRV
? The approach in APRV is to maintain lung
volume, improving both oxygenation and
ventilation.

• Reduce support through manipulation of PHigh and
  THigh
• Decrease PHigh 2 to 3 cm of water pressure at a
  time down to 14
• Lengthen THigh by 0.5- to 2.0-second increments,
  depending on patients tolerance up to 12-15
  seconds
• The goal is to arrive at straight CPAP usually
  at 12 cm of water pressure
• At 6 to 12 cm of water pressure either wean CPAP
  or extubate the patient
• Patients with more severe forms of ALI or ARDS
  are weaned on a slower basis
• Exhaled minute ventilation is tracked in
  conjunction with CO2 removal
• Changes in mean Paw are monitored closely for
  their effect on oxygenation.

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BiLevel Ventilation
PEEPH
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Thigh
Pressure
Tlow
Phigh
Plow
Time
Pressure
Psupp
Time
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Thigh
Pressure
Tlow
Phigh
Plow
Time
Pressure
Psupp
Phigh
Time
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Thigh
Pressure
Tlow
Phigh
Plow
Time
Pressure
Psupp
Psupp
Phigh
Time
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